Confused about dB's, Sensitivity....

[joel2762]

I was recently looking at a Decibel level chart. I tried to upload but it was too big. This chart states that 90 Db can be dangerous under constant exposure. Okay. So my speakers are rated at 89 dB/1 watt. Does this mean that at one watt from my reciever my speakers can almost damage my hearing!? What's all the extra power there for then?? I don't get this...There must be something more to it that I don't get here but i'd like to know what it is. Thanks in advance!

[This Guy]

After 8 hours 90 decibels will hurt your hearing. Most of your listening is done with one watt or less of power. And don't tell me you don't crank those speakers on occasion. Look at it this way, to go twice as loud (10 decibels) as 1 watt you need 10 watts, and to go twice as loud as that you need 100 watts. That will make your speakers 110 decibels loud with a 100 watts, at one meter. Subtract 6-8 dB because of dispersion (distance) and add maybe 2-3 decibels for room gain and your speakers are about 104 decibels max at your listening position. You can listen this loud for like an hour or so on occasion and get temporary hearing loss. If you do this every day you'll get permnent hearing loss is certain frequencies.

-Joey

[RGA]

Yes 1 watt will achieve 90db at 1 meter away. Most people listen 2-3 meters back and you can roughly subtract 3db with each meter.

Basically if you are seated 3 meters back you'd need about 4-10 watts to get 90db at your listening position. This is a gross generalization and depends on how big your room is and the type of speaker.

This is why the 10 Watt Sugden A21 sold from 1968-1989 and then when speakers became hard to drive Sugden used new better heat handling parts to increase the pure class A solid state amp to 25 Watts which quite happily drive the Totem Arro's 4ohm 86db sensitive at quite solid volume levels.

Watts are the biggest crapola spec on the market especially when the Sugden has bass control and depth with speakers that my old 125Watt Pioneer Elite Reciever would flab up and just plain suck the life out of music.

[poneal]

Well stated RGA! I get chagrined when people post that they need 200+ watts because it just dont play loud enough. I'm pretty sure if they use all that power on a regular basis they are having hearing loss.

[Woochifer]

I was recently looking at a Decibel level chart. I tried to upload but it was too big. This chart states that 90 Db can be dangerous under constant exposure. Okay. So my speakers are rated at 89 dB/1 watt. Does this mean that at one watt from my reciever my speakers can almost damage my hearing!? What's all the extra power there for then?? I don't get this...There must be something more to it that I don't get here but i'd like to know what it is. Thanks in advance!

You're learning! All that extra wattage makes for great marketing print, even if in reality the capability is never used. Also, keep in mind that those wattage figures,especially for multichannel amps is rarely if ever expressed with all channels driven. So, that 100 watt x 7 channel receiver you see advertised at most stores is not really a 700 watt receiver (the power consumption stat on the back of the unit is usually a dead giveaway that it's a physical impossibility for the unit to output 700 watts). In addition, there are myriad ways for stat sheets to distort the actual output figures, like posting output levels with high distortion or using low impedance tests. The thing to look for is a 20 Hz-20 kHz test done at 8 ohms impedance -- that's the standard FTC test used for mono and stereo amps.

As stated earlier, it's true that your speakers will output to very high levels with just one watt of output. The only time you'll really need a lot of wattage is if you're going with a low efficiency speaker or you're in a large room.

The perceived loudness basically doubles with every 10 db increase, but in order to get that doubled loudness, the wattage demand increases tenfold. Normal listening levels are about 75-85 db, typical speakers need less than one watt per channel to drive to that point. Getting that same speaker up to a concert level volume of 95-105 db would require between 10 and 100 watts per channel. That range is really where the extra wattage is needed. But, for everyday listening with relatively efficient speakers, the amp will make a minimal difference in the overall sound quality compared to differences between speakers and room acoustics.

[joel2762]

Thanks alot, I get it now. That's pretty interesting though because you'd think you'd need more than a watt to drive speakers loud. But I guess that's not true! My reciever manual states that the volume control increases by 1 dB each step. I listen to it around 45-50 depending on the level of the recording pretty much everyday. I don't crank it to dangerous levels, it'[s only a small room but every now and then on the weekend might put it a little louder for a little while. I just like bass, before I had my subwoofer, I could not achieve the bass I wanted from my speakers without having it too loud. Now I have a sub it all blends in.

[kexodusc]

This was a really good thread, I don't think this question gets asked enough, especially by newer audio enthusiasts or people looking to build their first systems.
I get people asking me all the time and I hear it at places like BestBuy and Sears, "is it worth that extra hundred bucks to get that extra 10-20 watts?"
They perceive the difference between 80 watt and 100 watt receivers as a factor of 20%...You wouldn't believe how many people drop 100-200 bucks extra here thinking they need it.
I always tell them that unless there's some other features that they absolutely need or like on the receiver, if everything else is equal, they've got better places to spend that 100-200 bucks on than going to 100 watts per channel. Geez, I love it when newbies look at Harmon Kardon receivers with 35 or 45 watts/channel and think "rip-off", then proceed to buy the 120 watt/channel Kenwoods.

Why would you drop 200 bucks for more power that isn't going to make a big impact and you don't really need?
Some brands are really bad, they up the power by 5 watts, add 1 more a/v connection, a DSP or two, and charge 150 bucks more for the receiver.

[magictooth]

Hi!

I thought that I should post a slightly contrarian point of view. While I agree with everything that the above posters have said, you should be aware that the extra watts are not absolutely wasted.

For the most part you have a desired listening level - say 75 dB. However, the one thing that the other posters haven't mentioned is the dynamic range that most movies have. You may like listening to conversation at 75 dB, but there will be times when you get a spike in volume level such as in an explosion or even something simple like a heated argument. At those times, you want to have the extra power available to fully express the dynamics in the soundtrack.

[kexodusc]

Hi!

I thought that I should post a slightly contrarian point of view. While I agree with everything that the above posters have said, you should be aware that the extra watts are not absolutely wasted.

For the most part you have a desired listening level - say 75 dB. However, the one thing that the other posters haven't mentioned is the dynamic range that most movies have. You may like listening to conversation at 75 dB, but there will be times when you get a spike in volume level such as in an explosion or even something simple like a heated argument. At those times, you want to have the extra power available to fully express the dynamics in the soundtrack.

That's a good point magictooth, and I agree with your logic, I'm just not convinced that a 5 or even 10 watt per channel difference will be of much value once you get over the 40 watt range for spikes in volume in most "moderate" sized home theater or stereo setups.
I'm only basing this on my past experience with 2 of my Marantz receivers...I ugraded to an extra 15 watts per channel from 90 to 105, and didn't notice a difference at all in performance or anything, and was kind of disappointed that I dropped money on the extra power after I realized I rarely ever come anywhere close to using it's full potential. I could be wrong because I never compared the receivers back and forth playing the same movie against each other, I was just expecting the extra power to improve sound quality or really shake the house.

[bfalls]

You're learning! All that extra wattage makes for great marketing print, even if in reality the capability is never used. Also, keep in mind that those wattage figures,especially for multichannel amps is rarely if ever expressed with all channels driven. So, that 100 watt x 7 channel receiver you see advertised at most stores is not really a 700 watt receiver (the power consumption stat on the back of the unit is usually a dead giveaway that it's a physical impossibility for the unit to output 700 watts). In addition, there are myriad ways for stat sheets to distort the actual output figures, like posting output levels with high distortion or using low impedance tests. The thing to look for is a 20 Hz-20 kHz test done at 8 ohms impedance -- that's the standard FTC test used for mono and stereo amps.

I'm not positive, but I don't think the power consumption of the receiver is rated the same as power provided to the speakers. Usually the receiver's power consumption is rated at it's input voltage (120VAC). The last I checked, output transistors (SS) and speakers don't run at 120V. Sensitivity is usually a decibel rating at 1W, 1M, 2.83V at either 1KHz or 20Hz-20KHz. Although the voltage won't remain constant 700W @120V is much different than 700W @ 2.83V.

Using a simple power formula (P=EI) a very rough calculation can be made for demonstration purposes I know it doesn't take into account speaker impedance, change due to frequency, or phase angle. Rated consumption power of 700W @ 120V results is 5.833A over the seven amp modules or .833A/ch. Well within the reach of an receiver's power supply to perform.

Using your assumption that the rating is the speaker power, 770W @ 2.83V results in 272A or 38.9A/ch. Within the reach of some higher-end amp capable of delivering 60+A to the output, but far from the norm.

I agree most HT receiver don't/can't output what their reported specs. It's been proven over and over. I just don't think the power consumption of the unit is all that telling of its performance.

What the "fuzz" MoviesOnlineTR? Is the TR short for TROLL?

[Chuck]

I was recently looking at a Decibel level chart. I tried to upload but it was too big. This chart states that 90 Db can be dangerous under constant exposure. Okay. So my speakers are rated at 89 dB/1 watt. Does this mean that at one watt from my reciever my speakers can almost damage my hearing!? What's all the extra power there for then?? I don't get this...There must be something more to it that I don't get here but i'd like to know what it is. Thanks in advance!

When a loudspeaker has a rating of 89 dB/watt/meter (which is probably the specification you're looking at) it means that the speaker will produce an output of 89 dB when one watt is applied and the SPL is measured at a distance of one meter.

Each time you double the distance you loose 3dB, so if you sit 2-meters from your speakers the output with one watt in would only be 86 dB.

Peak levels for an orchestra can exceed 102 dB, and for rock music peak levels can be much higher. "Spectaculars" like performances of the 1812 that use real cannons have even higher levels.

Each 3dB more output requires us to multiply the amplifier power by two. The following table applies.

86 dB = 1 watt
89 dB = 2 watts
92 dB = 4 watts
95 dB = 8 watts
98 dB = 16 watts
101 dB = 32 watts
104 dB = 34 watts

Classical music with peaks of 100+dB rarely have an average an level in excess of 80 to 85 dB, and it is the high averages that do the damage (in this range). At extremely high levels (like some live rock music) hearing damage happens very quickly. Keep the average as measured using an SPL meter below 90dB long term, but expect the musical peaks to require substantially more than one watt.

Hope this helps.

[jack70]

A couple of points that aren't being made clear here... (perhaps missed by some?):

Electrical Watts (amp power) and Audio Watts (acoustic energy that hits yer ears), are 2 different things.

Acoustic (perceived by our ears) energy is an "absolute thing". If you went into the quietest room in your house, you'd find the background sound level was around 30-40 db. That's an Absolute figure.

The Electrical energy of the amp is totally dependent on the efficiency of the speakers. You could have an amp that puts out 100 electrical watts through an 8-ohm speaker... this would be equal to 28 volts across it (sq rt of 100x8). If the speaker is very efficient it'll "sound" loud (acoustic energy). If it's very ineficient, it'll sound much quieter for the same input going into the amp. The amp-speaker interaction is the key here.

You could have a 10 watt amp conected to a speaker with 115db "efficiency" that puts out 125 db of acoustic energy (loudness to your ears). You could have a 500 watt amp connected to a low effiecient speaker (70db) that sounds much quieter in comparison.

Its' true that perceived "loudness" is a logrithmic (power) function... it requires "more & more" power to hear "a small amount" of loudness increase.

The other issues discussed (frequency aberations) and (peak power) are both more complex issues, because they depend on more variables. Generally, audiophiles use high power amps because they have the reserve electrical power to reproduce the kind of (very-small) peaks in reproduction that make music sound more real... these are instantaneous (fast) pulses of power... the kind you'd hear from the sound of a hammer or a gun. It's not a constant acoustic output, but of a short duration. It's impossible to reproduce that "reality", through any speaker with a smaller amp (all things considered). But having that extra "power reserve" of a big amp isn't valuable unless the speaker system is matched well to the electrical circuitry of that particular amp.

Speakers are VERY inefficient devices. They waste 90% (more or less) of the amps energy in mechanical ways. The acoustic power they put out is but a fraction of the electrical power coming into them. They also have dozens of times the distortion of the electrical signal coming into them. That's also why speakers are such a neat thing to play around with for most audiophiles -- they all have inherent defects and a "ways to go" to reach perfection.

[Chuck]

A couple of points that aren't being made clear here... (perhaps missed by some?):

Electrical Watts (amp power) and Audio Watts (acoustic energy that hits yer ears), are 2 different things.

Acoustic (perceived by our ears) energy is an "absolute thing". If you went into the quietest room in your house, you'd find the background sound level was around 30-40 db. That's an Absolute figure.

The Electrical energy of the amp is totally dependent on the efficiency of the speakers. You could have an amp that puts out 100 electrical watts through an 8-ohm speaker... this would be equal to 28 volts across it (sq rt of 100x8). If the speaker is very efficient it'll "sound" loud (acoustic energy). If it's very ineficient, it'll sound much quieter for the same input going into the amp. The amp-speaker interaction is the key here.

You could have a 10 watt amp conected to a speaker with 115db "efficiency" that puts out 125 db of acoustic energy (loudness to your ears). You could have a 500 watt amp connected to a low effiecient speaker (70db) that sounds much quieter in comparison.


Its' true that perceived "loudness" is a logrithmic (power) function... it requires "more & more" power to hear "a small amount" of loudness increase.

The other issues discussed (frequency aberations) and (peak power) are both more complex issues, because they depend on more variables. Generally, audiophiles use high power amps because they have the reserve electrical power to reproduce the kind of (very-small) peaks in reproduction that make music sound more real... these are instantaneous (fast) pulses of power... the kind you'd hear from the sound of a hammer or a gun. It's not a constant acoustic output, but of a short duration. It's impossible to reproduce that "reality", through any speaker with a smaller amp (all things considered). But having that extra "power reserve" of a big amp isn't valuable unless the speaker system is matched well to the electrical circuitry of that particular amp.

Speakers are VERY inefficient devices. They waste 90% (more or less) of the amps energy in mechanical ways. The acoustic power they put out is but a fraction of the electrical power coming into them. They also have dozens of times the distortion of the electrical signal coming into them. That's also why speakers are such a neat thing to play around with for most audiophiles -- they all have inherent defects and a "ways to go" to reach perfection.

Hi Jack,

You are correct in concluding that the loudspeaker and amp are key factors, but the size of the room and listening distances are equally important factors. You are also confusing "sensitivity" and "efficiency." Efficiency is always rated as a percentage. Sensitivity is a measure of pressure-level at a given distance using a specified input . The two are related, but VERY different.

Using acoustic watts as a measure of power needs isn't going to work very well. A 36-inch bass drum can produce a peak power of around 25 acoustic watts. Speaker efficiency is typically between 1% and 10% (can be lower or higher, but this is typical) so we'd need from 10 to 100 times that 25-watts just to produce the power of the 36-inch bass drum. That would be from 250 to 2500 watts! However, if you put that drum in your living room, along with the rest of the orchestra, and then have them play at peak levels, you'll be deaf in short order. In a huge performance hall the power is needed, but not in your home. If you take a sound system that can reach 100dB at Row M in Radio City Music Hall and put it in your bathroom it will produce a high enough pressure level (SPL) to exceed the pain threshold.

The industry settled on sensitivity measurements years ago because they are far more useful than efficiency numbers. As a system designer they allow us to *estimate* the output we can expect, and by applying a few "rules of thumb" we can easily insure that we'll be able to produce the target SPL levels.

Working with watts and efficiency really falls apart when we look at something like an ESL, as they are driven by voltage, and could be said to be "wattless speakers." Working with sensitivity allows consistency even with odd driver types. Your numbers (70dB and 115dB) for sensitivity aren't at all typical, but your point is valid. More commonly we might end up comparing a loudspeaker that has a sensitivity of 82dB/w/m with one that has a sensitivity of 97dB/w/m (the latter being the sensitivity of my ESL's). The former loudspeaker will only reach 102dB *peak* at one meter when driven by a 100 watt amplifier, but the latter will reach 107dB with only a 10 watt amplifier. If the goal is to produce 110dB peaks then the 82dB/w/m speaker will need well over that 100 watts, but the second will reach that level with only 20 watts.

Of course the above example also shows the danger in any over-simplification, because if you try to drive my stats with any commercial 20-watt amplifier it will most likely shut down instantly. ESL's present a nasty load to the amp, and the load is yet another factor to consider when determining amplifier requirements.

Rod Elliott, and others (most notably Bob Carver) have written extensively on amplifier requirements and works like the "Handbook for Sound Engineers" are full of info on determining the power requirements for almost any situation, and it is a topic that is far to complex to discuss on an audio forum. It takes chapters, and anybody with the level of interest to tackle all the reading and the math simply needs to get a few good books. Nobody want's to read an entire course on the topic in a string of forum posts.

Crest factor hasn't even been mentioned.

Without going into too many details, some of the important factors to consider are:

* Room size.
* Room acoustics (dead, live, etc.)
* Distance from source to listener.
* Loudspeaker sensitivity.
* Loudspeaker load (as seen by the amplifier).
* Types of music to be reproduced.
* Listener preferences (some like it louder than others).

Other things that most seem to overlook:

* Distortion levels - Sound can seem "ear splitting" at modest levels if there are objectionable distortion products. When someone says "my 30 watt amp is already WAY TOO LOUD when the volume control is at 3:00" it is almost always distortion giving them the impression of "TOO LOUD."

* Crest factor - This is a complex-signal issue. With even a slightly complex signal we have to consider crest factor. Two tones are enough. Assume we record 100 Hz. and 2000 Hz., and that each is originally produced by a 10 watt source. We have two 10 watt sources, but to reproduce the two tones with a single speaker and amplifier we need more than 20 watts. In fact, we need 40 watts, due to the crest factor of the two signals when they are mixed (the 2k signal rides on top of the 100 Hz. signal producing twice the voltage swing, and twice the voltage swing means four times the power. This is one of the reasons so many people go to bi-amping with their flagship loudspeaker designs, and why most pro loudspeakers are powered and bi-amped, but it is probably too complex to cover in forum posts. Check the literature (Rod has some good info on his site).

So, Jack, you are right even if a little confused over terms; A LOT has been glossed over here, but the basics still hold. If you take the sensitivity of a loudspeaker, adjust for the listening distance, ignore room support, and then use the desired peak SPL desired, you can calculate the amplifier power required. Good engineering practice requires that you add in a margin for error (at least 3dB) and I'd add a little more for other factors (like the crest factor). THEN I'd add a little more with an SS amp to make sure that protective circuits were either never partially triggered, or simply weren't needed.

I think the original post discussed 89dB/w/m. Here's how I'd *estimate* the amplifier requirements for my situation. I sit four meters from my speakers. With one watt I can expect a little over 83dB at the listening position (a little over due to room support) with one watt applied. There are times (RARE), when playing music with lots of bass, and showing off the system, that I've seen 112dB in my room, and I want at least 3dB margin, so I want to shoot for a peak output of 115dB. Assuming single amp and passive xovers we can now calculate the power we need to reach (about) 115dB.

1w = 83dB
10w = 93dB
100w = 103dB
1000w = 113dB
2000w = 116dB

(note that we have two speakers and two channels, so it would take almost 1000 wpc to reach that 115dB mark. AND, we still haven't considered the crest factor issue. How many speakers will even handle 1000 watts? If we add in 3dB more for to handle worse case crest factor issues, and another 3dB to alow a margin of error or to avoid partial protective circuit triggering, we're talking 4000 watts per channel!!!

Increasing the speaker sensitivity 10dB would reduce that by a factor of 10, to only 400 watts, so the speaker sensitivity is obviously a key factor.

Note that when the bass peaks are reaching 112dB, the average level is right around 90dB, and that the bulk of the high-energy is at the lower frequencies. If that weren't the case this level would be ear-splitting (and if the sound is distorted it will be ear-splitting at much lower levels). I use ESL panels that have a sensitivity of 97dB/w/m, and drive them with an amp that can deliver over 1000 watts to the load without working up a sweat. That's JUST for the mid-tweeter. The mid-woofer has it's own 600 wpc amp and the sub-woofer has it's own 1000+ watt amplifier. The system reproduces highly dynamic music at any desired level as effortlessly and with as much impact as any high-efficiency horn system (but with a much flatter response). Does ANYONE actually NEED that much power and output capability?
IMHO, ABSOLUTELY NOT.

However, 200wpc was NOT enough to drive my mid-woofers, and 400 watts was NOT enough to drive my sub, even at more normal (and common) "audiophile" listening levels (averages approaching 90dB). With the smaller amps the sound was LOUD but distorted due to all the momentary clipping on peaks. Not obvious to anyone who has never heard a system that is free of such problems, but very obvious when compared to the same drivers with larger amps.

Velodyne upped their sub drives from 400 watts to over 1000 watts, because 400 just wasn't enough. Ask them if you don't believe me.

InnerSound upped the power of the amp driving their mid-woofers from 200 watts to 600 watts, because 200 just wasn't enough. Ask them if you don't believe me.

Kilpsch Corner Horns can be driven to INSANE levels with a 10 watt amplifier, so the speaker certainly can make all the difference.

I use to sacrifice efficiency (as well as sensitivity and high output capability) to get lower distortion, wider bandwidth, and excellent transient characteristics, but that is no longer necessary. Today we can have our cake and eat it too (as long as money and floor space isn't an issue).

Yea, Jack, we glossed over a BUNCH of stuff, but I don't think any thing that was missed was actually relevant to the original question.

If I understood it properly, the original question could be rephrased this way: "If my speakers have a sensitivity of 89dB/watt/meter and 90dB will damage my hearing, then why would I want an amplifier larger than 2 watts per channel?" I think I answered that question in my response. If he's putting the system in a bathroom he'll need less power, in Radio City Music Hall he'll need a lot more, but in general, if he takes the sensitivity and adjusts for distance and desired SPL he'll see why he needs more than 1-watt. That's all he asked. Why more than a watt or two? Didn't I answer that fully enough?

(I could have just said "crest factor, distance, and a bunch of other stuff" but the explanation of sensitivity and distance seemed most appropriate. Do you think I was mistaken?

See ya,

Chuck